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Md#P2URD20N09501A Ser#EHEM339385
Two pipe system. Problem is when burners first ignite it seems that pressure builds up inside burner box, causing flames to go on and off rapidly, sounding like a machine gun. Pressure switch that connects to collector box flutters as this is happening.
Switch is rated at .40WC. Checked and pressure seems to start at .40 and flutters some, then as burners smooth out goes up to .60WC. Gas pressure is 3.4WC. Removed exhaust pipe from top of furnace, pressure switch wouldn't even close. Doesn't seem to be restriction in pipe,and pipe seems to be properly pitched toward furnace.
This one has me stumped. I jumpered switch, burners still ignite poorly. Not as bad. Any help would be appreciated.

Check your intake pipe. In the larger models we had to drill holes in the burner box for this reason or take the rubber out that enclosed the gas line. Another problem with this furnace is the gasket on the ventor will move up and block the hole. How it moves up I have no idea but we have had it happen several times.

When I took off intake pipe pressure switch would not close either. Flame looks fine once it settles down. As far as the ventor being partially blocked by gasket, when this happened, did the burners continue to run or did it shut down on pressure switch opening?
Thanks for the input.

I had a similar problem with a york 90+ on new const. install. Turned ot to be drain related. I had to install an air gap at the drain outlet on the side of the furnace. Replace 90 with a tee and put a short peice of pipe in the top of the tee. hope this helps.

When the gas valve opens it changes the pressure in the sealed combustion chamber and thats why your getting this fluttering of the pressure switch. You are running to close to the switch setting before the vave opens and after the pressure of the gas comes into the burner box its changing your pressures on the diff. switch. You have a problem with the vent system somewhere. Try taking the intake and exhaust pipes off and see what it does. Check the gasket on the ventor blower and the plastic drain pan that the ventor bolts to is bad about cracking also. If this all checks out then the ventor is weak, you have a plugged secondary(could be water), or the heat exchanger has failed.

Thanks for the info. The drain seemed to be draining fine but I did not take it apart because I really didn't think a plugged drain would cause this kind of reaction. I did notice though that tha collector looked new, like it was too clean for this old of a furnace. As far as I know our company is the only one servicing this unit and there was no record of this being changed. If there was a crack in the heat exchanger, do they normally crack right at the top so you can see it if you remove the burner box and manifold?
I will check the amp draw on the inducer.
Thanks again guys

I have not had one of these furnaces that the heat exchanger failed so I can't tell you where to look. I have a lot of these furnaces out and really don't have to much trouble with them. The worst problem is water leaking from the seal on the drain pan or from it being cracked.

I forgot to say that water standing in the secondary could be your problem and you also might check the pressure switch stub that comes off the left lower side of the secondary drain pan. We have had these plug before.

check the drain tube from the bottom of secondary coil drain fitting, to the trap (a short formed tube). It can create a water trap if not pushed down all the way on to the trap fitting. Also., make sure you have the correct orifices @ press. switch for altitude. Hope this helps.